The present invention relates to a method for manufacturing field-effect transistors constructed entirely by thin-film techniques. The invention also relates to a field-effect transistor constructed entirely by thin-film techniques.
A realization of thin-film integrated circuits has recently been required as it has been one of important subject matters to fabricate a transistor by the thin-film techniques. The transistor of this type is known as a thin-film transistor (TFT) and functions as a switching element or active circuit element in the above thin-film integrated circuit. In case that a thin-film formed of a semiconductor material is formed at a relatively low temperature on the top surface of a substrate, the material for the substrate has a large selection, and a thin-film IC pattern forming method is known to have such advantages that the pattern can be readily formed in accordance with conventional exposure techniques and etching techniques.
Since the TFT is commonly formed on the top surface of the substrate with polycrystalline semiconductor or amorphous semiconductor material, the carrier mobility thereof becomes lower than that of the single crystalline semiconductor material. This is particularly remarkable in the TFT formed with amorphous semiconductor material. Therefore, the TFT formed with polycrystalline or amorphous semiconductor material has considerably narrow operation frequency range as compared with that of the ordinary MOSFET. Further, the operation frequency range of the TFT is narrowed and the operating speed is undesirably lowered by the adverse influence of the storage capacitance or parasitic capacitance produced in a transistor structure and the wiring pattern formed on the substrate.